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Understanding phase changes is crucial when examining substances such as iodine (I2), which can transition from a solid to a gas. A phase change occurs when a substance changes its physical state—think of ice melting into water or water boiling into steam. These transformations require energy, which is associated with the internal energy of the substance.

During a phase change, the temperature of the substance remains constant while absorbing or releasing heat, known as latent heat. This heat goes into altering the physical structure without changing the temperature. For example, the transition from solid iodine to gaseous iodine involves a phase change where iodine undergoes sublimation—a direct shift from a solid to a gas without passing through a liquid state.

The enthalpy change (ΔH) is related to the heat content of the system. It's the amount of heat absorbed or released during a chemical reaction or a phase change under constant pressure. In our exercise, to calculate the temperature at which iodine changes from solid to gas (sublimation), we use the enthalpy of sublimation, which we represent as 'X' kJ/mol.

Enthalpy change is essential because it helps predict whether a process is endothermic (absorbs heat) or exothermic (releases heat). For sublimation, this value is typically positive, reflecting the energy required to overcome intermolecular forces within the solid iodine and convert it into gaseous form.

The concept of entropy is closely linked to disorder or randomness in a system. The entropy change (ΔS) during a phase change measures the change in disorder when a substance transforms from one phase to another. Considering our iodine example, the transition from a solid (ordered state) to a gas (disordered state) increases the system's entropy.

Understanding entropy is key to determining the spontaneity of a process. A higher entropy signifies more disorder, which nature tends to favor. The increase in entropy is quantified in units of J/mol·K, and for iodine's phase change, we represent it as 'Y' J/mol·K. This entropy change figures into the temperature calculation we conduct to estimate the point of zero free-energy change for iodine's sublimation.

In the context of phase changes, like the sublimation of iodine, chemical equilibrium refers to the state where the rate of the forward reaction (solid to gas) equals the rate of the reverse reaction (gas to solid). When the free-energy change (ΔG) for a reaction is zero, the system is at equilibrium, indicating that the two phases — solid and gas iodine, in this case — coexist in balance.

At equilibrium, the concentrations of reactants and products remain constant over time, although they may not be equal. This dynamic balance is framed by the principle of Le Chatelier, which states that the system will adjust to minimize changes when subject to external stress, such as temperature or pressure variations. Hence, understanding chemical equilibrium is vital for manipulating reactions and phase changes in practical applications.